THE ENVTBONMENT OF BACTERIA. 77 



Moreover, direct experiment has shown that many bacteria 

 can multiply under a pressure of several hundred atmos- 

 pheres. Of course a pressure of several hundred atmos- 

 pheres to the square inch becomes reduced, owing to the 

 extremely small size of bacteria, to but a few milligrams 

 for each individual cell. 



Electricity. 



It is very difficult to demonstrate a direct action of 

 electricity on bacteria. This is due to the fact that an 

 electric current induces chemical and physical changes in 

 the liquid. The chemical changes are especially marked 

 in solutions of various salts. Thus, if common salt is 

 present it may not only decompose into acid and alkali but 

 it may also give rise to free chlorine and to hypochlorites. 

 The bacteria suspended in a liquid containing salt may be 

 destroyed as the result of the passage of an electric cur- 

 rent, but the immediate cause of death is the formation of the 

 germicidal products mentioned above. Moreover, hydrogen 

 peroxide, and especially ozone, may form and destroy the 

 bacteria present. The purification of polluted river-water 

 by means of ozone generated by powerful currents of elec- 

 tricity has been carried on successfully. 



In addition to the chemical changes caused by an 

 electric current a considerable rise in temperature may be 

 observed. Obviously, the heat thus generated will not be 

 without effect upon the organisms in suspension. 



It is not possible to overcome, or to do away with the 

 action of the chemical products and of heat when studying 

 the action of an electric current. It would seem as if the 

 latter possessed little or no direct action. A feeble gal- 

 vanic current has little or no effect, whereas a strong cur- 

 rent, especially if allowed to act for some hours, will destroy 

 the organisms present. That this result is due to an alter- • 



